Syria's government pounded rebel-held Aleppo with air strikes and artillery fire on Saturday, killing 31 civilians and prompting Washington to condemn the "heinous actions" of Damascus and its ally Russia.

UN officials said they were "appalled" by escalating violence and urged access to east Aleppo, where more than 250,000 people have been under siege for nearly four months.

Since it began on Tuesday, the assault on the rebel-held east of Aleppo has damaged hospitals, forced schools to close, and killed nearly 100 civilians, according to a monitor.

An AFP correspondent described relentless bombardment with air strikes, mortar rounds and barrel bombs hitting neighbourhoods where residents cowered at home.

"It is a catastrophic day in besieged Aleppo with unprecedented bombardment with every type of weapon," a member of the White Helmets rescue group said in a video on the organisation's Facebook page.

US National Security Advisor Susan Rice said Washington condemned "in the strongest terms these horrific attacks against medical infrastructure and humanitarian aid workers".

"There is no excuse for these heinous actions," she said.

"The Syrian regime and its allies, Russia in particular, bears responsibly for the immediate and long-term consequences these actions have caused in Syria and beyond."

Moscow says it is not involved in the current assault on Aleppo, concentrating its firepower on opposition and jihadist forces in neighbouring Idlib instead.

But Damascus and its allies have made clear they want rebels expelled from eastern Aleppo, which fell from regime control in mid-2012.

- UN officials 'appalled' -

Once Syria's economic powerhouse, Aleppo has been ravaged by the war that has killed more than 300,000 people since it began with anti-government protests in March 2011.

No aid has entered east Aleppo since government forces surrounded it in July, and residents report food, fuel and water shortages and lengthy power cuts.

Two top UN officials said they were "extremely saddened and appalled by the recent escalation in fighting in several parts of Syria".

Humanitarian coordinator for Syria Ali al-Za'atari and regional humanitarian coordinator Kevin Kennedy also said they had shared a plan to deliver aid, and evacuate the sick and wounded from east Aleppo.

"It is imperative all parties agree to the plan and allow us to secure immediate, safe and unimpeded access to provide relief to those most in need," they said.

The relentless bombardment forced schools in east Aleppo, many of which already operate from basements because of government attacks, to close Saturday and Sunday, a statement said, "for the safety of students and teachers, after the barbarous aerial strikes".

Hospitals and rescue facilities have been particularly affected, with shelling destroying one of the last medical facilities in the east on Friday.

Staff were also forced to evacuate the east's only children's hospital because of repeated attacks, removing babies from incubators.

"This is a dark day for east Aleppo," said Teresa Sancristoval, emergency coordinator for the Doctors Without Borders (MSF) charity.

"The severity of the bombing has inflicted huge damage on the few hospitals working around the clock to provide medical care."

- 'No more body bags' -

A White Helmets centre in the Bab al-Nayrab district was destroyed in an air strike on Friday, and rescuers have struggled to cope with the pace of the bombing.

"We have no more (body) bags," said one rescue worker in another video posted Saturday.

The Observatory said at least 31 civilians were killed in east Aleppo on Saturday, including five children.

It also said 17 men who were not identified were killed.

The deaths brought the death toll since Tuesday to 96, with state media saying two people were also killed in rebel fire on the government-held west of Aleppo on Saturday.

The assault on rebel-held Aleppo has ended a period of relative respite after Russia halted its strikes and organised a series of brief truces to encourage residents and surrendering rebels to leave.

Syria expert Thomas Pierret said regime forces "intended to combine air strikes with famine resulting from the siege to get rebels to surrender".

Elsewhere, an alliance of Kurdish and Arab fighters known as the Syrian Democratic Forces said they had seized the hilltop village of Tal Saman from the Islamic State group.

The village is 25 kilometres (15 miles) from IS's de facto Syrian capital of Raqa, which the SDF began an operation to capture earlier this month with support from the US-led anti-IS coalition.

The Observatory reported seven civilians killed in an air strike believed to have been carried out by the coalition on Buaas village in northern Raqa province.

source: AFP